This invention relates generally to current regulators and, more particularly, to current regulators for programmable logic controllers (PLCs).
PLC's commonly utilize analog current regulator outputs for signaling and/or control purposes. In a typical PLC, an output stage of a linear circuit dissipates power thermally and regulates current by modulating a voltage drop caused by a loaded device in the output stage. Sometimes, a design goal will require a specified number of current outputs in a single PLC package. However, the number of current outputs is sometimes limited by the amount of power dissipated thermally. In a linear circuit, the power dissipated is the product of the current through the circuit and the voltage drop across the circuit. One known method for reducing the amount of power dissipated in a linear circuit is to replace the linear circuit with a pulse width modulated (PWM) circuit. In a PWM circuit, the current is pulsed through a saturated switch and, therefore, the voltage drop across the switch is kept low, which results in a lower product of the circuit current and voltage drop across the circuit. However, known methods and apparatus for controlling current output with a PWM current regulator only allow an output current to be variable over an eight to one range i.e., from about 150 mA to 1200 mA.
Accordingly it would be desirable to provide a PWM power output circuit for a PLC that dissipates less power than a linear circuit dissipates and allows an output current to be variable over a range greater than eight to one.